1. Field of the Invention
The present invention relates to an image sensor of a multi-chip type having a plurality of photoelectric conversion chips.
2. Related Background Art
Linear image sensors have been used as a reading apparatus of a facsimile, a scanner or the like. Photoelectric conversion chips of a linear image sensor are formed on a silicon wafer. A length of the linear image sensor is therefore limited by the size of a wafer, and in many cases, only short-length liner image sensors can be formed. In this case, a reading apparatus using only one photoelectric conversion chip reduces an image of light reflected from an original by using an optical system and projects it on the chip to read the image. If such a reduction optical system is used, it is necessary to reserve a large space of the optical system and the size of the reading apparatus cannot be made compact. In order to solve this problem, a multi-chip type image sensor has been proposed having a plurality of photoelectric conversion chips disposed linearly.
Techniques regarding this field are disclosed, for example, in the publications of JP-A-6-189065 and JP-A9-205588.
FIG. 1 is a block diagram showing a conventional multi-chip type image sensor using a plurality of photoelectric conversion chips. FIG. 2 is a circuit diagram of one pixel of the multi-chip type image sensor. FIG. 3 is a timing chart illustrating the operation of the multi-chip type image sensor.
In FIG. 1, reference numeral 1 represents a photoelectric conversion chip. A plurality of photoelectric conversion chips are mounted on a substrate and connected electrically to form a multi-chip type image sensor. Reference numeral 2 represents a photoelectric conversion element for converting an input photo-signal into an electric charge signal. Photoelectric conversion chips shown in FIG. 1 are herein called a first photoelectric conversion chip, a second photoelectric conversion chip, . . . , beginning with the leftmost chip. It is assumed that each photoelectric conversion chip has n pixels or n photoelectric conversion elements which are called herein from the leftmost element a first photoelectric conversion element, a second photoelectric conversion element, . . . By disposing a plurality of photoelectric conversion elements, the multi-chip type image sensor can have a large effective read width and can process a large size original.
A timing circuit 10 generates control signals for the image sensor operation by receiving a clock signal 111 and a start signal 122.
When a MOS switch 4 turns on in response to a photo-signal read pulse, the photo-signal is read out and stored in a storage capacitor 6. The photo-signal is obtained by voltage-amplifying an electric charge signal converted by the photoelectric conversion element 2 by a source follower circuit 3 constituted of transistors 3a and 3b.
Thereafter, the photoelectric conversion element 2 is reset to start again storing the photo-signal.
At the same time when this storage operation starts, a shift register 7 sequentially turns on a MOS switch 5 synchronously with the clock signal 122 to read the photo-signal from the storage capacitor 6 and output it via an output buffer 9 to an output terminal 13.
After the photo-signal is read and output and when the start signal is supplied from the external, the next cycle of the photo-signal reading and storing starts and the above-described operation is repeated.
Each photoelectric conversion chip 1 has a constant current circuit 8 for controlling the current flowing through the source follower circuit 3. If all the source follower circuits of all chips are made fully operable while the photo-signal is read from each photoelectric conversion chip, the total consumption current becomes large. In order to avoid this, the consumption current flowing through the photoelectric conversion chips other than the chip whose photo-signal is presently read is suppressed by limiting the current of the constant current sources circuits.
A photo-signal output from the conventional multi-chip type image-sensor constructed as above contains random noises and fixed pattern noises (FPNs) generated in the photoelectric conversion elements and source follower circuits in addition to random noises and FPNs generated in the constant current source circuits. Noises generated in the constant current source circuit affect all other photoelectric conversion elements in the same photoelectric conversion chip, and the photo-signal varies with each photoelectric conversion chip because there is a variation in the characteristics of the chips. Therefore, when such photo-signals are displayed as an image, vertical or horizontal stripes appear on the photoelectric conversion chip basis. These stripes are very easy to be visually recognized, which becomes a main factor of a degraded image quality. This phenomenon is inherent to multi-chip type image sensors.